I think you mean to say the the expansion slots on your case is limited to only one while you have a dual slot GPU, correct? If that's the case your only solution would be to either get a case that has the necessary expansion slots or to mod the case(which means cutting it up), which I'm not comfortable in suggesting you do.
if you think there is an error in understanding/miscommunication, you'd be best served by passing a picture of the issue and a link to the card you own.